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Marquee University e-Publications@Marquee Physical erapy Faculty Research and Publications Physical erapy, Department of 11-1-2012 Influence of Knee Flexion Angle and Age on Triceps Surae Muscle Fatigue During Heel Raises Kim Hébert-Losier University of Otago Anthony G. Schneiders University of Otago José A. García University of Otago S. John Sullivan University of Otago Guy G. Simoneau Marquee University, [email protected] Accepted version. Journal of Strength and Conditioning Research, Vol. 26, No. 11 (November 2012): 3124-3133. DOI. © 2012 Lippinco Williams & Wilkins, Inc. Used with permission.

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Page 1: Influence of Knee Flexion Angle and Age on Triceps Surae

Marquette Universitye-Publications@Marquette

Physical Therapy Faculty Research and Publications Physical Therapy, Department of

11-1-2012

Influence of Knee Flexion Angle and Age onTriceps Surae Muscle Fatigue During Heel RaisesKim Hébert-LosierUniversity of Otago

Anthony G. SchneidersUniversity of Otago

José A. GarcíaUniversity of Otago

S. John SullivanUniversity of Otago

Guy G. SimoneauMarquette University, [email protected]

Accepted version. Journal of Strength and Conditioning Research, Vol. 26, No. 11 (November 2012):3124-3133. DOI. © 2012 Lippincott Williams & Wilkins, Inc. Used with permission.

Page 2: Influence of Knee Flexion Angle and Age on Triceps Surae

NOT THE PUBLISHED VERSION; this is the author’s final, peer-reviewed manuscript. The published version may be accessed by following the link in the citation at the bottom of the page.

Journal of Strength and Conditioning Research, Vol. 26, No. 11 (November 2012): pg. 3134-3147. DOI. This article is © Lippincott Williams & Wilkins, Inc. and permission has been granted for this version to appear in e-Publications@Marquette. Lippincott Williams & Wilkins, Inc. does not grant permission for this article to be further copied/distributed or hosted elsewhere without the express permission from Lippincott Williams & Wilkins, Inc.

1

Influence of Knee Flexion Angle and

Age on Triceps Surae Muscle Activity

during Heel Raises

Kim Hébert-Losier Center for Physiotherapy Research, University of Otago

Dunedin, New Zealand

Anthony G. Schneiders Center for Physiotherapy Research, University of Otago

Dunedin, New Zealand

José A. Garcia Department of Preventive and Social Medicine

University of Otago

Dunedin, New Zealand

John S. Sullivan Center for Physiotherapy Research, University of Otago

Dunedin, New Zealand

Guy G. Simoneau Department of Physical Therapy, Marquette University

Milwaukee, WI

Abstract: Hébert-Losier, K, Schneiders, AG, García, JA, Sullivan, SJ, and

Simoneau, GG. Influence of knee flexion angle and age on triceps surae

muscle activity during heel raises. J Strength Cond Res 26(11): 3124–3133,

2012—Triceps surae and Achilles tendon injuries are frequent in sports

medicine, particularly in middle-aged adults. Muscle imbalances and

weakness are suggested to be involved in the etiology of these conditions,

Page 3: Influence of Knee Flexion Angle and Age on Triceps Surae

NOT THE PUBLISHED VERSION; this is the author’s final, peer-reviewed manuscript. The published version may be accessed by following the link in the citation at the bottom of the page.

Journal of Strength and Conditioning Research, Vol. 26, No. 11 (November 2012): pg. 3134-3147. DOI. This article is © Lippincott Williams & Wilkins, Inc. and permission has been granted for this version to appear in e-Publications@Marquette. Lippincott Williams & Wilkins, Inc. does not grant permission for this article to be further copied/distributed or hosted elsewhere without the express permission from Lippincott Williams & Wilkins, Inc.

2

with heel-raise testing often used to assess and treat triceps surae (TS)

injuries. Although heel raises are recommended with the knee straight for

gastrocnemius and bent for soleus (SOL), the extent of muscle selectivity in

these positions is not clear. This study aimed to determine the influence of

knee angle and age on TS muscle activity during heel raises. Forty-eight

healthy men and women were recruited from a younger-aged (18–25 years)

and middle-aged (35–45 years) population. All the subjects performed

unilateral heel raises in 0° and 45° knee flexion (KF). Soleus, gastrocnemius

medialis (GM) and gastrocnemius lateralis (GL) surface electromyography

signals were processed to compute root-mean-square amplitudes, and data

were analyzed using mixed-effects models and stepwise regression. The mean

TS activity during heel raises was 23% of maximum voluntary isometric

contraction when performed in 0° KF and 21% when in 45°. Amplitudes were

significantly different between TS muscles (p < 0.001) and KF angles (p <

0.001), with a significant interaction (p < 0.001). However, the age of the

population did not influence the results (p = 0.193). The findings demonstrate

that SOL activity was 4% greater when tested in 45° compared with 0° KF

and 5% lower in the GM and GL. The results are consistent with the

recommended use of heel raises in select knee positions for assessing,

training, and rehabilitating the SOL and gastrocnemius muscles; however, the

4–5% documented change in activity might not be enough to significantly

influence clinical outcome measures or muscle-specific benefits. Contrary to

expectations, TS activity did not distinguish between middle-aged and

younger-aged adults, despite the higher injury prevalence in middle age.

Introduction

Muscle and tendon injuries are common in sport participants.

Muscle strains account for 10–55% of all injuries occurring during

sporting events (14,21), whereas 30–50% of overuse injuries are

tendinous (22). The triceps surae (TS) muscles and Achilles tendon

(AT) are reported as some of the most frequently injured structures in

sport (13,30), and specific clinical procedures are used to manage

and prevent these conditions. For example, repetitive concentric-

eccentric heel-raise performances are employed to assess the TS and

are included in exercise regimens to condition, rehabilitate, and in

prevention of AT and TS injuries (5,41).

Task specificity is an important concept in muscle rehabilitation,

strengthening, and conditioning (11). For the TS muscles, heel raises

are traditionally prescribed with 0° knee flexion (KF) to target the

gastrocnemius medialis (GM) and gastrocnemius lateralis (GL), and

45° KF for the soleus (SOL) (1,20). The muscle bias associated with

heel-raise repetitions is proposed to change with KF angle and is

Page 4: Influence of Knee Flexion Angle and Age on Triceps Surae

NOT THE PUBLISHED VERSION; this is the author’s final, peer-reviewed manuscript. The published version may be accessed by following the link in the citation at the bottom of the page.

Journal of Strength and Conditioning Research, Vol. 26, No. 11 (November 2012): pg. 3134-3147. DOI. This article is © Lippincott Williams & Wilkins, Inc. and permission has been granted for this version to appear in e-Publications@Marquette. Lippincott Williams & Wilkins, Inc. does not grant permission for this article to be further copied/distributed or hosted elsewhere without the express permission from Lippincott Williams & Wilkins, Inc.

3

based on the structural anatomy, architecture, and muscle function of

the TS muscles (40). To date, research also suggests that heel raises

performed in different knee positions influences TS muscle specificity.

For instance, Price et al. (32) used electromyography (EMG) in

conjunction with magnetic resonance imaging techniques to

investigate the TS muscles during dynamic plantar-flexion

contractions at 3 KF angles: 0° KF, 45° KF, and 90° KF. These

authors reported decreasing GM and GL activities with increasing SOL

activity as KF was increased. Although the latter findings support the

use of different KF angles during heel raises to target the individual

TS muscles, they derive from a non–weight-bearing task (sitting) and

generalization to the conventionally prescribed heel-raise procedures

in weight bearing (standing) is not appropriate. International research

priorities include validating assessment procedures commonly used in

musculoskeletal practice (37), and to date, there is currently limited

evidence to justify using change in KF to direct heel-raise testing or

conditioning toward the SOL or GM and GL. Therefore, the specificity

of this task to the different TS muscles remains questionable.

Some musculoskeletal injuries are more frequent in younger-

aged adults, but the prevalence of TS and AT injuries is reported to

increase significantly after the mid-20s (31) and is highest in the

fourth and fifth decades of life (23). Increase in age and muscle

imbalances or weakness are recognized as predisposing factor to TS

and AT pathogenesis (5), and it is important to identify risk factors or

abnormal musculoskeletal functions to decrease the prevalence of

injury. Surface EMG is a noninvasive technique often employed in

clinical settings to explore muscle function (8) and to detect

alterations, weakness, pathology, and potential injury risk factors

(42). Whether an alteration in TS muscle function in middle-aged

adults during heel raises can be captured through EMG has not yet

been determined and may assist healthcare professionals and

strength and conditioning coaches in further understanding injury

prevalence, specifying risk factors, and establishing tailored

management strategies specific to population age groups.

In consideration of the above clinical observations, the aim of

this study was to investigate the influence of performing heel raises in

Page 5: Influence of Knee Flexion Angle and Age on Triceps Surae

NOT THE PUBLISHED VERSION; this is the author’s final, peer-reviewed manuscript. The published version may be accessed by following the link in the citation at the bottom of the page.

Journal of Strength and Conditioning Research, Vol. 26, No. 11 (November 2012): pg. 3134-3147. DOI. This article is © Lippincott Williams & Wilkins, Inc. and permission has been granted for this version to appear in e-Publications@Marquette. Lippincott Williams & Wilkins, Inc. does not grant permission for this article to be further copied/distributed or hosted elsewhere without the express permission from Lippincott Williams & Wilkins, Inc.

4

0° KF and 45° KF on the amount of SOL, GM, and GL activities, and

the effect of age on the activity of the TS muscles. The formulated

hypotheses were based on the traditional use of heel raises in select

KF positions and its applications in musculoskeletal practice. More

precisely, the hypothesis was that SOL activity would be greater

during heel raises in 45° KF than in 0° KF, with greater GM and GL

activities in 0° KF compared with 45° KF. In a second instance, it was

speculated that TS activity from middle-aged individuals would be

different from that of younger-aged adults in that the proportional

contribution of each TS muscle toward the task would differ. An

additional aim was to explore whether muscle activity differed

between the concentric and eccentric plantar-flexion contractions of

heel raises, considering their role in injury management and sports

training. The hypothesis was that the amount of activity recorded

during concentric contractions would be greater than that from

eccentric contractions considering their lower physiological efficiency

and reported lower activity levels at given workloads (7).

Methods

Experimental Approach to the Problem

This study used a prospective repeated-measures design and

surface EMG techniques to investigate SOL, GM, and GL activities

during heel raises. The root-mean-square (rms) amplitude variable

can be derived from EMG signals to quantify the amount of muscle

activity and is suggested to provide valuable information on muscle

force generation and contribution to movement (27). The rms

amplitude expressed as percent of maximum voluntary isometric

contraction (MVIC) was hence chosen as the primary dependent

variable. To investigate the muscle specificity of the heel-raise task

with change in the KF angle, the 3 TS muscles (SOL, GM, and GL)

were considered as separate entities and 2 KF positions were chosen

for testing. Heel raises in a knee straight position of 0° KF and in a

knee bent position of 45° KF were selected to reflect those described

in clinical practice and used for GM and GL, and SOL, respectively. In

addition, the recruitment process targeted equal numbers of younger-

aged (18–25 years) and middle-aged (35–45 years) individuals to

explore whether TS muscle activity during heel raises was different

Page 6: Influence of Knee Flexion Angle and Age on Triceps Surae

NOT THE PUBLISHED VERSION; this is the author’s final, peer-reviewed manuscript. The published version may be accessed by following the link in the citation at the bottom of the page.

Journal of Strength and Conditioning Research, Vol. 26, No. 11 (November 2012): pg. 3134-3147. DOI. This article is © Lippincott Williams & Wilkins, Inc. and permission has been granted for this version to appear in e-Publications@Marquette. Lippincott Williams & Wilkins, Inc. does not grant permission for this article to be further copied/distributed or hosted elsewhere without the express permission from Lippincott Williams & Wilkins, Inc.

5

between populations with a relatively low and high prevalence of AT

and TS muscle pathologies, respectively. Finally, the concentric and

eccentric components of a heel-raise cycle were delineated and used

to explore differences between the 2 types of contractions per the

additional research aim. In summary, the key dependent variable was

the rms amplitude expressed as percentage of MVIC and the grouped

independent variables inclusive of: 3 TS muscles, 2 KF positions, 2

age groups, and 2 types of contractions.

Subjects

Forty-eight volunteers were recruited from the local community

and took part in this study. Subject numbers for men and women

were balanced to control for the potential confounding effect of

gender on EMG signal. All the subjects were tested within a 2-month

period and at a time of the day that was convenient to them. They

were asked to refrain from strenuous activities for 24 hours before

study participation. The individuals were included if they reported

being in good general health and were excluded if they reported a

current or previous injury to the AT or TS muscles, or a medical

condition that could limit their ability to perform unilateral heel

raises. All the subjects were provided with an information sheet of

the study purpose and procedures and signed an informed consent

document on the day of testing. The research was approved by the

university's Institutional Review Board before subject recruitment

and testing, which was held in an EMG-biomechanics laboratory.

Baseline measures of the subjects were collected on the day of

testing and included the Dunedin Footedness Inventory to determine

lower-limb dominance (39) and the self-administered short-form

International Physical Activity Questionnaire to capture physical

activity levels and training backgrounds (6). None of the subjects

were professional athletes. Baseline characteristics of the 48 subjects

are summarized in Table 1.

Procedures

Each subject performed 1 trial of 10 unilateral heel raises with

their dominant lower limb in both 0° KF and 45° KF, with the aligned

femur and tibia defining 0° KF. The 2 selected KF angles were chosen

Page 7: Influence of Knee Flexion Angle and Age on Triceps Surae

NOT THE PUBLISHED VERSION; this is the author’s final, peer-reviewed manuscript. The published version may be accessed by following the link in the citation at the bottom of the page.

Journal of Strength and Conditioning Research, Vol. 26, No. 11 (November 2012): pg. 3134-3147. DOI. This article is © Lippincott Williams & Wilkins, Inc. and permission has been granted for this version to appear in e-Publications@Marquette. Lippincott Williams & Wilkins, Inc. does not grant permission for this article to be further copied/distributed or hosted elsewhere without the express permission from Lippincott Williams & Wilkins, Inc.

6

based on common clinical musculoskeletal assessment procedures

(20), strength training protocols prescribed in rehabilitation (1), and

EMG studies of selective TS muscle activation in various KF positions

(40). To account for the possible effects of testing sequence, the

order of the 0° KF and 45° KF trials was block randomized according

to age group and gender before the study. To position the subjects'

knee in either 0° KF or 45° KF, the principal investigator (K.H.-L.)

followed standard goniometry guidelines (34) and used a long-arm

goniometer (Fred Sammons Inc.; Brookfield, IL, USA). An

TequipmentTequipment goniometer (Noraxon USA Inc.; Scottsdale,

AZ, USA) was also fixed to the lateral aspect of the subjects' lower

limb using double-sided tape with the fulcrum aligned with the knee-

joint center, proximal arm with the greater trochanter, and distal arm

with the lateral malleolus. The angular readings (degrees) from the

electromechanical goniometer were displayed in real-time on a

computer screen to the investigator who could then monitor KF

angles and provide verbal information to subjects during heel-raise

testing. If the KF angles were on average >±5° of the target angle

during testing, the subjects were rested for 5 minutes and asked to

repeat the trial. A maximum of 3 trials was permitted per KF angle,

and the one with the lowest absolute angular error was subsequently

retained for further analysis. Illustrations of the 2 KF angles and a

complete heel-raise trial are provided in Figure 1.

A custom-made adjustable frame with an integrated

dynamometer (Chatillon CSD200 Series Dynamometer, Ametek Inc.;

Largo, FL, USA) was used for balance support. The subjects were

allowed to place their index fingertips on the dynamometer handles

positioned at shoulder height in front of them and use a maximum

force equivalent to 2% of their body weight (36). The threshold value

was individually established before heel-raise testing, and the

subjects used the displayed force readings from the dynamometer to

self-monitor the applied force. Stance foot placement was

standardized by aligning the first metatarsal to a predetermined floor

marking. Because TS muscle activity during dynamic tasks may be

influenced by foot placement (35), the subjects served as their own

controls by using their habitual unilateral weight-bearing stance for

testing. The nontested lower limb was free in space in a self-selected

position that allowed lower-limb clearance. An audible digital

metronome (Sabine MT9000, Sabine Inc.; Alachua, FL, USA) was set

Page 8: Influence of Knee Flexion Angle and Age on Triceps Surae

NOT THE PUBLISHED VERSION; this is the author’s final, peer-reviewed manuscript. The published version may be accessed by following the link in the citation at the bottom of the page.

Journal of Strength and Conditioning Research, Vol. 26, No. 11 (November 2012): pg. 3134-3147. DOI. This article is © Lippincott Williams & Wilkins, Inc. and permission has been granted for this version to appear in e-Publications@Marquette. Lippincott Williams & Wilkins, Inc. does not grant permission for this article to be further copied/distributed or hosted elsewhere without the express permission from Lippincott Williams & Wilkins, Inc.

7

at 120 b·min-1 to standardize the pace at 60 heel raises per minute;

the heel lifting to maximal heel-raise height on the first beat and

lowering to the floor on the second. One complete heel raise was

defined by 2 consecutive heel-to-floor contacts and included a data

point when the heel was at maximal height during that raise as

identified in Figure 1. The chosen heel-raise parameters for balance

support, body position, and pace were based on those most

frequently cited in the literature (15) and used in a previous study

(17). An overview of subjects' position and apparatus used for heel-

raise testing is provided in Figure 2.

The subjects were instructed that testing required them to

perform approximately 10 unilateral heel raises while maintaining the

selected KF angle as precisely as possible. The subjects lifted the heel

as high as possible during each heel raise and return the heel to the

floor after each heel raise in pace with the metronome beats. They

were reminded that the nontested limb should not contact the floor

and to use the frame for balance support only. A countdown directed

the subjects to start, and 15 seconds later, an audible signal advised

the subjects to stop. The 15-second interval was selected to allow for

the: initiation of the first heel raise; habituation of subjects to heel

raise pace; and collection of data from 10 consecutive heel raises.

To describe, monitor, and relate the EMG data to heel-raise

performance, kinematic data of lower-limb motion were acquired

using a 3D motion analysis system incorporating 12 calibrated

optoelectric cameras sampling at 100 Hz (Cortex version 1.1.4.368

software, Motion Analysis Corporation, Eagle EGL-500RT; Santa Rosa,

CA, USA). Although a full series of retroreflective movement markers

were used, only sagittal-plane data from select markers were

extracted for the purpose of this study. The angle data recorded from

the markers placed over the lateral malleolus, lateral femoral condyle,

and greater trochanter were used to represent knee angles in

degrees, and the distance between the marker positioned on the

posterior aspect of the heel and the ground calibration markers was

computed to provide a measure of heel-raise height in millimeters.

The EMG signals from SOL, GM, and GL were recorded via

surface electrodes. Skin preparation and electrode positioning

followed the Surface EMG for the Noninvasive Assessment of Muscles

(19) and International Society of Electrophysiology and Kinesiology

Page 9: Influence of Knee Flexion Angle and Age on Triceps Surae

NOT THE PUBLISHED VERSION; this is the author’s final, peer-reviewed manuscript. The published version may be accessed by following the link in the citation at the bottom of the page.

Journal of Strength and Conditioning Research, Vol. 26, No. 11 (November 2012): pg. 3134-3147. DOI. This article is © Lippincott Williams & Wilkins, Inc. and permission has been granted for this version to appear in e-Publications@Marquette. Lippincott Williams & Wilkins, Inc. does not grant permission for this article to be further copied/distributed or hosted elsewhere without the express permission from Lippincott Williams & Wilkins, Inc.

8

(26) recommendations. Subjects' skin was shaved, lightly abraded,

and cleaned with alcohol swabs to reduce tissue impedance and

electrode-to-skin artifacts. To correctly position the electrodes, all

muscle bellies were first identified by palpation while under

contraction. Electrode locations were specified and paired Ag-AgCl

gelled electrodes (Ambu Blue Sensor SP, AMBU A/S; Ballerup,

Denmark) were subsequently placed on the non-contracted muscle

bellies of (a) SOL, approximately two-thirds of the distance between

the medial femoral condyle and medial malleolus; (b) GM, on the

upper third of the posterior leg over the maximal muscle bulge on the

medial side; and (c) GL, similar to GM, but on the lateral side of the

leg. All paired electrodes were trimmed to allow interelectrode

distances to be <20 mm and were oriented parallel to the underlying

muscle fibers. A single ground electrode was positioned over the tibial

tuberosity. Skin impedance was measured using a multimeter (Fluke

70 series II, Tequipment.NET; Long Branch, NJ, USA) and the skin

preparation process was repeated until all electrode sites were

<3,000 [OMEGA]. The EMG signals were collected at a sampling rate

of 3,000 Hz using a Noraxon TeleMyo 2400 T G2 and MyoResearch XP

Master Edition Software package (Noraxon USA Inc., Scottsdale, AZ,

USA). The telemetered system applied a band-pass filter of 10–500

Hz; no notch (50/60 Hz) filters; input impedance >100 M[OMEGA]; a

common mode rejection ratio >100 dB; a baseline AC noise <1 µV

rms; an input range ±3.5 mV; and a gain of 1,000. The data were

transmitted and digitized with 16-bit resolution for all analog inputs

using a series of 30 data points for the zero correction of all acquired

signals. A digital synchronization pulse was employed to allow for the

combination and the synchronization of the collected EMG and

kinematic data sets.

The experimental flow of the laboratory testing session was as

follows: On the day of testing, the subjects signed an informed

consent document and were then familiarized with testing equipment,

procedures, and protocols. Baseline characteristics of the subjects

were recorded, and lower-limb dominance was determined. The EMG

recording system and the electromechanical goniometer were

positioned, and signal quality was verified. If the acquired signals

contained artifacts, the setup was adjusted until the quality of signals

was appropriate. The MVIC data were captured for EMG normalization

and heel-raise experimentation followed.

Page 10: Influence of Knee Flexion Angle and Age on Triceps Surae

NOT THE PUBLISHED VERSION; this is the author’s final, peer-reviewed manuscript. The published version may be accessed by following the link in the citation at the bottom of the page.

Journal of Strength and Conditioning Research, Vol. 26, No. 11 (November 2012): pg. 3134-3147. DOI. This article is © Lippincott Williams & Wilkins, Inc. and permission has been granted for this version to appear in e-Publications@Marquette. Lippincott Williams & Wilkins, Inc. does not grant permission for this article to be further copied/distributed or hosted elsewhere without the express permission from Lippincott Williams & Wilkins, Inc.

9

The activity of the TS muscles during an MVIC was captured

during an externally resisted unilateral heel raise performed to

maximal height with the use of a Smith machine (Multipower M053

Technogym; Gambettola, Forli, Italy). A detailed account of the MVIC

procedural methods employed has been published elsewhere (16).

Three MVIC trials of 7 seconds, interspersed with an appropriate rest

period of 2 minutes, were performed in each of the following KF

positions: 0° KF (standing), 45° KF (standing), and 90° KF (sitting).

The respective peak amplitudes captured from the SOL, GM, and GL

during a stable 3 seconds from any 1 MVIC trial was extracted and

used for subsequent normalization.

After MVIC data collection, the retroreflective movement

markers were placed, and a static motion image was recorded to

define the initial position of the markers. The standardized heel-raise

instructions were provided, and the subjects performed a trial of heel

raises in either 0° KF or 45° KF according to the preallocated random

testing sequence. A 5-minute rest period was allocated, the

instructions were reiterated, and the subjects performed another trial

of 10 unilateral heel raises in the other KF position. During testing,

the investigator provided consistent verbal feedback to ensure that all

the instructions and parameters were followed and observed the

angular readings provided by the electromechanical goniometer.

Additional trials were performed if required.

Data Processing

The kinematic data collected during heel raises were referenced

to the captured baseline static motion image. The first 2 heel raises

completed were excluded to eliminate possible end-point errors, and

the following 5 were extracted for further analyses (17,35). A

schematic summary of the data reduction process is illustrated in

Figure 1. The average knee angle maintained (degrees), maximum

heel-raise height (millimeters), and mechanical work (joules) to

complete a heel raise were computed to extract clinically meaningful

biomechanical performance measures. Work was calculated by using

the subject's weight (kilograms), average maximum heel-raise height,

and gravitational acceleration (9.81 m·s-2) (41).

Each complete heel raise captured 1 full TS muscle contraction

and included the concentric and eccentric components. The maximum

Page 11: Influence of Knee Flexion Angle and Age on Triceps Surae

NOT THE PUBLISHED VERSION; this is the author’s final, peer-reviewed manuscript. The published version may be accessed by following the link in the citation at the bottom of the page.

Journal of Strength and Conditioning Research, Vol. 26, No. 11 (November 2012): pg. 3134-3147. DOI. This article is © Lippincott Williams & Wilkins, Inc. and permission has been granted for this version to appear in e-Publications@Marquette. Lippincott Williams & Wilkins, Inc. does not grant permission for this article to be further copied/distributed or hosted elsewhere without the express permission from Lippincott Williams & Wilkins, Inc.

10

height data point from each heel raise was used to separate the 2

types of contraction, where concentric occurred from the first heel-to-

floor contact to the maximum heel-raise height data point and

eccentric from the maximum heel-raise height data point to the

second heel-to-floor contact (Figure 1).

The EMG signals of each extracted subset of 5 heel raises were

processed to compute the SOL, GM, and GL rms amplitudes

(microvolts) in 250-millisecond epochs. The derived rms amplitudes

from each muscle were normalized to their respective peak

amplitudes captured during MVIC procedures and expressed as

percent of MVIC. For additional descriptive information regarding the

overall activity of the TS muscles, the average percent activity of the

SOL, GM, and GL from each trial was calculated to provide a

numerical representation of the mean amount of average TS (ATS)

muscle activity (29). For quality assurance of the surface EMG data

muscle selectivity, a crosscorrelation processing technique was

applied to the signals collected during MVIC and determined that

crosstalk was minimal (8).

Statistical Analyses

Sample size determination was based on the assumption that

the activity ratios between the SOL and GM would differ in the 2 KF

positions. Pilot study data and EMG data from research studies with

similar aims (40,43) were used to obtain estimates of means and

variability in activity ratios in 0° KF and 45° KF. Sample size

requirements were calculated using standard 2-tailed hypothesis

equations, the derived estimates, an [alpha] = 0.05 (5% significance

level), a [beta] = 0.10 (90% power), critical values of the t-

distribution, and adjustments for 20% missing data (46). Calculations

determined that a minimum sample of 44 subjects was required. To

balance the study design and block randomize the order of testing

according to age group and gender, sample size requirement was

increased to 48 to have 12 men and 12 women in the 2 age groups.

The literature reports adequate to excellent reliability of the test

measurements used in this study; hence, reliability data were not

collected. The intraclass correlation coefficients reported in the

literature for heel-raise performance range from 0.78 to 0.99 (9,28),

kinematic variables from similar motion analysis methods range from

Page 12: Influence of Knee Flexion Angle and Age on Triceps Surae

NOT THE PUBLISHED VERSION; this is the author’s final, peer-reviewed manuscript. The published version may be accessed by following the link in the citation at the bottom of the page.

Journal of Strength and Conditioning Research, Vol. 26, No. 11 (November 2012): pg. 3134-3147. DOI. This article is © Lippincott Williams & Wilkins, Inc. and permission has been granted for this version to appear in e-Publications@Marquette. Lippincott Williams & Wilkins, Inc. does not grant permission for this article to be further copied/distributed or hosted elsewhere without the express permission from Lippincott Williams & Wilkins, Inc.

11

0.89 to 0.92 (12), and muscle activity data from the TS muscles

during dynamic tasks from 0.82 to 0.99 (25,44).

Mixed-effects models and stepwise regressions were used for

the main analysis. This statistical approach allowed us to determine

whether the variables of interest influenced the amount of muscle

activity recorded during heel raises and to derive estimates of the

difference between testing conditions. The mixed-effects model

considered the influence of the individual TS muscles (SOL, GM, GL),

KF angles (0° KF, 45° KF), and age groups (younger, middle) on rms

signal amplitudes expressed as percent of MVIC. The model also

considered all potential interaction effects. The model applied a

generalized Gaussian distribution (Normal), adequately accounted for

the repeated-measures design of the study by clustering within-

subject observations (random effect), and assumed an equal amount

of correlation between subjects' measures (exchangeable correlation).

Nonsignificant effects were sequentially removed from the

original model through stepwise regression according to the Bayesian

information criterion, which penalizes a model according to its number

of parameters (4). The nonsignificant 3-way and 2-way interactions

were removed first. The final model from the stepwise analysis

contained only effects that significantly influenced outcomes. The

intercept and coefficients from the final regression were extracted and

reported to provide values to estimate TS muscle EMG signal

amplitudes during heel raises performed under a known set of

conditions. The model intercept value provides an estimate of the

amplitude compared to the defined conditional reference of SOL, 0°

KF, and younger-age group. For instance, the KF coefficient from the

final regression provides an estimate of the difference in amplitudes if

heel raises are performed in 45° KF instead of 0° KF with all the other

conditions remaining constant.

To see whether muscle activity differed between the concentric

and eccentric contractions of heel raises, a similar rationale was

employed. However, the mixed-effects model additionally included

contraction (concentric, eccentric) and its interactions with KF,

muscle, and age group. The concentric contraction was used as the

reference in the final regression of the stepwise analysis, with

coefficients estimating the difference in eccentric amplitudes

compared to concentric.

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NOT THE PUBLISHED VERSION; this is the author’s final, peer-reviewed manuscript. The published version may be accessed by following the link in the citation at the bottom of the page.

Journal of Strength and Conditioning Research, Vol. 26, No. 11 (November 2012): pg. 3134-3147. DOI. This article is © Lippincott Williams & Wilkins, Inc. and permission has been granted for this version to appear in e-Publications@Marquette. Lippincott Williams & Wilkins, Inc. does not grant permission for this article to be further copied/distributed or hosted elsewhere without the express permission from Lippincott Williams & Wilkins, Inc.

12

The probability level was set at 0.05 before all analyses, which

were controlled for the repeated-measures design of the study.

Because the influence from muscle was of threefold, Tukey Contrasts

were used for multiple comparisons with the Westfall method

adjusting the 95% confidence interval (CI) and p values accordingly

(3,45). All data processing and analyses were completed using R™

version 2.12.2 (R Foundation for Statistical Computing™; Vienna,

Austria).

Results

A pooled descriptive summary of the biomechanical measures

from heel-raise performances is provided in Table 2. The mean knee

angle maintained during 0° KF testing was 4.4° and in 45° KF was

45.3°. The angles maintained were ±5° from the starting angle in

both KF test positions.

A combined summary of the EMG data collected from all the

subjects during heel raises is given in Table 3. The mean signal

amplitude in 0° KF compared with 45° KF was approximately 4% less

for SOL and 5% more for the GM and GL. The GM was the most

active muscle during 0° KF heel raises; SOL, when in 45° KF; with GL

the least active in both positions. The mean amount of ATS activity

was 2% higher in 0° KF (23%) compared with 45° KF (21%).

Crosstalk was considered negligible because all absolute mean and

peak values derived from the crosscorrelation of signals were <0.30

(8).

The main findings from the stepwise regression of EMG signal

amplitudes during heel raises are given in Table 4. Amplitudes were

significantly different between the SOL, GM, and GL muscles (p <

0.001), and in the 2 KF positions (p < 0.001). The interaction

between TS muscle and KF angle also significantly influenced

amplitudes (p < 0.001). However, no significant differences between

age groups were determined (95%CI: -0.9–4.2; p = 0.193) with all

other possible interactions having no significant influence on muscle

activity during heel raises (p >= 0.05). The effects from age group

and all their interactions were therefore dropped from the model

during stepwise regression procedures. Because no significant

differences in amplitudes were determined between age groups,

detailed results are not included.

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13

The mixed-effects model used to derive coefficients for

estimating amplitudes considered: muscle, KF, and the interaction

between muscle and KF. As shown in Table 4, the difference in

amplitudes amongst the TS muscles during 0° KF heel raises was

only significant when comparing GL to GM (p = 0.018) and not when

comparing GM or GL with SOL (p > 0.142). The opposite was

evidenced in 45° KF where the difference in amplitudes was not

significant between GL and GM (p = 0.097) but was when comparing

the SOL with GM (p < 0.001) or GL (p < 0.001). A visual

representation of these results is provided in Figure 3 with the top

third (KF on muscle) illustrating that the effect of change in the knee

angle on SOL amplitude was different from its effect on GM and GL

amplitudes. The mean and 95%CI error bar of the difference in SOL

amplitude do not overlap with the ones from the GM or GL. In

contrast, the effect of change in the KF position on the GM and GL is

similar as indicates their overlapping 95%CI error bars.

The amount of TS muscle activity during heel raises can be

estimated for an individual by using the values given in Table 4. To

estimate GM amplitude during 45° KF heel raises, the following

rationale can be employed referencing the values from top to bottom

of Table 4: (a) the model intercept (23.4%) represents the activity of

SOL in 0° KF; (b) the influence of KF on SOL (3.6%) considers that

testing is in 45° KF and not 0° KF; and (c) the influence of muscle in

45° KF (-7.4%) takes into account that the estimate is for the GM and

not for the SOL. Therefore, the equation estimates a GM amplitude of

23.4 + 3.6 -7.4% = 19.6% during heel raises in 45° KF. The final

estimate corresponds to the sampled cohort mean value given in

Table 2 and is applicable to both younger and middle-aged adults.

The secondary analysis determined that EMG amplitudes were

in general 2.3% greater during eccentric contractions of heel raises

compared with concentric (95% CI: 0.8–3.7; p = 0.002). The type of

contraction did not interact with any of the other effects considered (p

>= 0.05), indicating that the amplitudes during concentric and

eccentric contractions were not significantly different between the 3

TS muscles, 2 KF positions, and 2 age groups. Therefore, only a

descriptive summary of amplitudes specific to concentric and eccentric

contractions is provided as a supplementary file (Supplementary

Table 2).

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14

Discussion

Understanding TS muscle activity during heel-raise performance

in specific KF angles has important clinical implications in the

assessment and rehabilitation of lower-limb injuries. The findings of

this research generally support the common muscle-selective use of

heel raises and accept the primary research hypothesis: SOL was

more active in 45° KF than in 0° KF, and vice versa for GM and GL.

However, it is not clear whether the reported changes in amplitudes

are clinically meaningful or large enough to influence the TS muscle

selectivity during heel-raise testing, rehabilitation, or conditioning.

Moreover, the differences in TS activity between middle-aged and

younger-aged individuals were anticipated, but none were observed

and the study hypothesis could not be accepted. No relationship

between TS activity and injury risk based on age group can be

established from these study results. Furthermore, the findings

demonstrated greater amounts of TS activity during eccentric rather

than concentric plantar-flexion contractions, which also contrasted

with the working research hypothesis. Implications from the study

findings are sequentially addressed from KF angles to age groups to

concentric-eccentric contractions.

This study reports the mean differences in the SOL, GM, and GL

amplitudes during heel raises that ranged from 3.6 to 5.5% when the

KF angles were changed. Previous studies report that the differences

in amplitudes that are <=5% of an MVIC are not likely to influence

clinical outcomes or rehabilitation (38), whereas a 20% variation in

amplitudes or more corresponds to a quantifiable change in function

and muscle performance scores (2). Based on these reports, the

current findings on the influence of KF angle on TS muscle activity are

not of clinical significance. The highest mean variation between

testing angles in a given muscle was 5.5% (GM) and between 2

muscles was 9.1% (GM and SOL). This suggests that coaches,

trainers, and healthcare professionals may choose either 0° KF or 45°

KF to assess, train, or rehabilitate SOL, GM, and GL by heel-raise

repetitions and expect to have similar clinical benefits and TS activity

levels at both angles.

However, although a change in KF angle may have a limited

clinical effect with respect to TS muscle activity during heel raises, the

findings indicate that it might with regard to other biomechanical

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15

characteristics and functional requirements. For instance, slightly

greater ATS muscle activity, mechanical work per heel raise, and

heel-raise height were noted in 0° KF compared with 45° KF.

According to a previous work (10), these changes in performance are

most likely associated with variations in AT moment arm, TS muscle

leverage, and plantar-flexion torque rather than in selective TS

muscle activity. These biomechanical alterations between KF positions

may lead to distinct structural loading patterns during heel raises,

which is an important factor during rehabilitation (33). It appears

more appropriate for clinicians to perceive heel raises in select KF

positions as influencing the architectural features, fascicle behaviors,

contributing muscle fibers, and loading patterns of the TS muscles

and AT rather than targeting the SOL in 45° KF and the

gastrocnemius in 0° KF.

In contrast to expectations, the age groups documented to

have a low (18–25 years) and high (35–45 years) prevalence of AT

and TS injuries showed no differences in the activity amplitudes and

individual contributions of the SOL, GM, and GL to heel-raise

performances. These findings suggest that, in asymptomatic

individuals, TS muscle activity quantified by surface EMG is not likely

to identify middle-aged individuals during repeated heel raises despite

the high prevalence of injuries reported in this population (23). The

results do not support the use of EMG for this purpose in clinical

practice. More plausible rationales of the higher prevalence include

age-related degeneration, degree of tissue compliance, morphologic

and histological compositions, and muscle timing or synchronization.

Our study identified higher levels of activity during eccentric

compared with concentric heel-raise contractions, contradicting our

initial research hypothesis. Determining the significance of these

results is difficult considering that the 2% difference has a limited

clinical value and that research comparing concentric and eccentric

contractions provide contrasting results (18,33). Omitting differences

in the EMG methodological procedures, the variable findings in the

literature are most likely related to the parameters dictating heel-

raise performance, for instance, pace. Henriksen et al. (18) reported

higher concentric compared with eccentric activity when subjects

performed 1 heel raise every 6.6 seconds. This study found 1ower

concentric than eccentric activity when using a quicker cadence.

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16

Faster heel raises might further rely on the storage and release of

elastic energy of the stretch-shortening cycle, which is characterized

by low concentric activity and shown in other EMG investigations (24).

This highlights how change in any given parameter can influence

muscle performance measures, which needs to be carefully

considered during clinical assessment and exercise prescription.

There are always certain methodological considerations with all

research projects. This study used surface EMG that is, at best, only

quasispecific to deep and overlaid muscles such as the TS (8).

However, we are confident that the data are as representative of SOL,

GM, and GL as permitted given that rigorous methodological EMG

procedures were followed and crosscorrelation analyses confirmed

minimal crosstalk. Surface EMG has the advantage of providing a

global functional representation of muscles, is not limited to a few

motor units, and is promoted for use in clinical practice (7).

Furthermore, the balanced design of this study, with its equal

numbers of male and female participants in 2 distinct age groups,

makes the study methodologically robust; especially when taking into

account its relatively large sample size compared with EMG studies of

a similar design (18,33).

Practical Applications

Heel raises are frequently used for the clinical management and

conditioning of the TS muscles and AT and recommended in 0° KF

and 45° KF for GM and GL, and SOL, respectively (1,20). The SOL,

GM, and GL were 4–5% more active in their respective recommended

KF angles, which is consistent with common musculoskeletal

assessment and training protocols. However, the extent of variation

might be too small to imply or influence TS muscle selectivity during

heel raises, which is consistent with earlier work indicating no

differences in total heel-raise numbers completed in different KF

positions (17). In general, the 3 muscles are clinically considered to

be just as active with the knee straight as bent, which implies that

either position is appropriate for targeting SOL, GM, or GL activities in

clinical practice. This suggests that using just 1 KF position may

increase the overall clinical efficiency of the heel-raise task without

significantly decreasing its efficacy in regards to specific TS muscle

activity. The influence of knee angles on TS muscle fatigue during

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17

heel raises in the 2 traditionally employed KF angles, and with

different amounts of resistance, could also be investigated to justify

the continued use of repeated heel-raises in select knee angles for

muscle-selective purposes in sports medicine and rehabilitation.

Acknowledgments

The authors acknowledge the grant funding support that was provided by the

Mark Steptoe Memorial Trust Research Grant-In-Aid from the School of

Physiotherapy, University of Otago, Dunedin, New Zealand. No other grant

funding was sought for the purpose of this research project. There are no

conflicts of interest to declare.

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Key Words: electromyography; lower limb; ankle plantar flexion; clinical

assessment; biomechanics

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21

Image Gallery

Figure 1. Representation of the 2 KF angle positions (top left), concentric and eccentric plantar-flexion contractions (top right), and a complete heel-raise trial with the data reduction process (bottom). The HF and MH data points, end-point errors, and 5 heel raises extracted for analysis in the early and late phases are identified. KF

= knee flexion; HF = heel to floor; MH = maximum heel-raise height.

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22

Figure 2. Subject during a heel raise performed in 0° KF. Testing apparatus and instrumentation are identified. KF = knee flexion.

Figure 3. Experimental flow of the laboratory testing session for a subject. MVIC =

maximum voluntary isometric contraction; KF= knee flexion

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23

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Journal of Strength and Conditioning Research, Vol. 26, No. 11 (November 2012): pg. 3134-3147. DOI. This article is © Lippincott Williams & Wilkins, Inc. and permission has been granted for this version to appear in e-Publications@Marquette. Lippincott Williams & Wilkins, Inc. does not grant permission for this article to be further copied/distributed or hosted elsewhere without the express permission from Lippincott Williams & Wilkins, Inc.

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Page 26: Influence of Knee Flexion Angle and Age on Triceps Surae

NOT THE PUBLISHED VERSION; this is the author’s final, peer-reviewed manuscript. The published version may be accessed by following the link in the citation at the bottom of the page.

Journal of Strength and Conditioning Research, Vol. 26, No. 11 (November 2012): pg. 3134-3147. DOI. This article is © Lippincott Williams & Wilkins, Inc. and permission has been granted for this version to appear in e-Publications@Marquette. Lippincott Williams & Wilkins, Inc. does not grant permission for this article to be further copied/distributed or hosted elsewhere without the express permission from Lippincott Williams & Wilkins, Inc.

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Figure 4. Estimates of the differences in MF (A–C) and MF-NS (D) between TS muscles. Interaction effects of muscle with KF (A), phase (B), and age (C) are represented. Full circles are means (•), and black lines are 95% confidence intervals

(–). The reference used for comparison is in parentheses. Comparison (reference) = comparison reference; EMG = electromyography; TS = triceps surae; MF = median

frequency; KF = knee flexion; SOL = soleus; GM = gastrocnemius medialis; GL = gastrocnemius lateralis; YA = younger-aged group of 18–25 years; MA = middle-aged group of 35–45 years; MF-NS = median frequency normalized slope. A). Estimate of the difference in MF between one TS muscle in a given KF position and another muscle and the other KF position during heel raises to fatigue. The effect of KF between muscles is the change in MF from 0° KF to 45° KF in 1 muscle compared with another

muscle. B) Estimates of the differences in MF between 1 TS muscle in a given phase and another muscle and the other phase during heel raises to fatigue. The effect of phase between muscles is the shift in MF from early to late in 1 muscle compared with another muscle. C) Estimates of the differences in MF between 1 TS muscle in a given age group and another muscle and the other age group during heel raises to fatigue. The effect of age between muscles is the change in MF from YA to MA in 1 muscle compared with another muscle. D) Estimates of the differences in MF-NS between 1

TS muscle and another muscle during heel raises to fatigue.

Page 27: Influence of Knee Flexion Angle and Age on Triceps Surae

NOT THE PUBLISHED VERSION; this is the author’s final, peer-reviewed manuscript. The published version may be accessed by following the link in the citation at the bottom of the page.

Journal of Strength and Conditioning Research, Vol. 26, No. 11 (November 2012): pg. 3134-3147. DOI. This article is © Lippincott Williams & Wilkins, Inc. and permission has been granted for this version to appear in e-Publications@Marquette. Lippincott Williams & Wilkins, Inc. does not grant permission for this article to be further copied/distributed or hosted elsewhere without the express permission from Lippincott Williams & Wilkins, Inc.

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